ai-bookkeeper was a web page. It is now also an installable app: "Add to Home Screen" on Android or iOS, or "Install" in a desktop browser, gives it its own icon and window.
Because the whole product runs in the browser, offline costs nothing extra. A service worker keeps the page and the 47 modules it imports; with the network off, the page reloads and every control is there. Your bank file is never uploaded and the worker only touches files from its own site, so the optional bring-your-own-key AI call still goes straight to your provider.
A test now fails if a module the page imports is missing from the offline list. What is not done: it has been checked in headless Chromium, not on a physical phone, and it is not in an app store. Your books live in the browser's local storage, so download the books file regularly.